In copending U.S. application 481,433 filed June 20, 1974, it is disclosed how a stepped diameter piston can sequentially produce a first pressure force and a second pressure force when a transfer control valve is operated by movement of a plunger connected to an input push rod. The input push rod transfers an output force created by the development of a pressure differential across a wall. The stepped diameter piston is connected to the wall. When the transfer control valve is activated, the output force is shifted from acting on a first diameter to act on the second diameter to produce an added output potential. However, in the event that the pressure differential across the wall does not develop, the transfer control valve directly shifts the input force from the first diameter to the second diameter completely eliminating the development of the first pressure force. In most applications, the development of the first pressure force is necessary to supply a braking system with sufficient fluid to allow full development of the second pressure force.